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Nonlinear dynamics in RFP experiment and computation

Author: Karsten J. McCollam
Requested Type: Consider for Invited
Submitted: 2012-12-07 16:20:17

Co-authors: J.A.Reusch, J.S.Sarff

Contact Info:
University of Wisconsin
1150 University Ave
Madison, WI   53706
USA

Abstract Text:
The nonlinear interaction of various physical effects in a complex dynamical system is studied using the RFP, which in the standard case exhibits a quasiperiodic sawtooth magnetic-relaxation cycle, or limit cycle. MST experiments have now been simulated in nonlinear, single-fluid, resistive MHD with the experimental Lundquist number, using the DEBS code [1]. Both the qualitative dynamical behavior of the equilibrium and tearing modes and the time scales of sawtooth periods are reproduced, which implies that the sawtooth phenomenon is a single-fluid effect. However, some features are not reproduced quantitatively, such as core mode amplitudes, which are about twice as large in the simulation as in the experiment. Additional effects, such as the Hall term and ion gyroviscosity used in some recent two-fluid MHD simulations [2] with the NIMROD code, may be needed to resolve such discrepancies. Meanwhile, oscillating-field current-drive (OFCD) experiments [3] with AC loop voltages provide both a probe of RFP dynamics and some control over them. Linear stability appears to be different than in the standard case, and the relaxation cycle can be drastically modified and tearing modes suppressed or excited, depending on the amplitudes and phases of the applied AC fields. As with the standard RFP, DEBS simulations of actual OFCD experiments reproduce them qualitatively and in some aspects quantitatively. These efforts, both for the standard and OFCD cases, suggest a program of validation and verification for single-fluid, resistive MHD and also for extended models.

[1] J. A. Reusch, J.K. Anderson, D.J. Den Hartog, F. Ebrahimi, D.D. Schnack, H.D. Stephens, C.B. Forest, PRL 107, 155002 (2011).

[2] J.R. King, C.R. Sovinec, V.V. Mirnov, POP 19, 055905 (2012).

[3] K.J. McCollam, J.K. Anderson, A.P. Blair, D. Craig, D.J. Den Hartog, F. Ebrahimi, R. O'Connell, J.A. Reusch, J.S. Sarff, H.D. Stephens, D.R. Stone, D.L. Brower, B.H. Deng, W.X. Ding, POP 17, 082506 (2010).

Characterization: 1.0,2.0

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